Decoration of Inorganic Nanostructures by Metallic Nanoparticles to

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Decoration of Inorganic Nanostructures by Metallic Nanoparticles to Induce Fluorescence, Enhance Solubility, and for Bandgap Tuning Priyadarshi Ranjan, Sreejith Shankar, Ronit Popovitz-Biro, Sidney R Cohen, Ifat Kaplan-Ashiri, Tali Dadosh, Linda J. W. Shimon, Bojana Visic, Reshef Tenne, Michal Lahav, and Milko E. van der Boom J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00510 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on February 27, 2018

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The Journal of Physical Chemistry

Decoration of Inorganic Nanostructures by Metallic Nanoparticles to Induce Fluorescence, Enhance Solubility, and for Bandgap Tuning Priyadarshi Ranjan†,‡, Sreejith Shankar†,¶, Ronit Popovitz-Biro§, Sidney R. Cohen§, Ifat Kaplan-Ashiri§, Tali Dadosh§, Linda J. W. Shimon§, Bojana Visic‡, Reshef Tenne‡,*, Michal Lahav†, and Milko E. van der Boom†,* †

Department of Organic Chemistry, The Weizmann Institute of Science, 7610001 Rehovot, Israel Department of Materials and Interfaces, The Weizmann Institute of Science, 7610001 Rehovot, Israel § Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 7610001, Israel ‡

ABSTRACT: We report here a unique and efficient methodology for the surface functionalization of closed-cage (fullerene-like-IF) nanoparticles and inorganic nanotubes (INT) composed of two-dimensional (2D) nanomaterials of transition-metal chalcogenides (MS 2 ; M = W or Mo). The first step is the physical coverage of these robust inorganic materials with monodispersed and dense monolayers of gold, silver, and palladium nanoparticles. The structural continuity at the interface between the IF/INT and the coinage metallic nanoparticles, are investigated. Lattice matching between these nanocrystalline materials and strong chemical affinity, leads to efficient binding of the metallic nanoparticles onto the outer sulfide layer of the MS 2 -based structures. It is shown that this functionalization result in narrowing of the IF/INT optical bandgap, increased work function, and improved surface-enhanced Raman scattering (SERS). In the second step, functionalization of the surface-bound nanoparticles by a ligand exchange reaction is carriedout. This ligand exchange involving the tetraoctylammonium bromide (TOAB) capping layer and an alkylthiol enhances the solubility (~10×) of the otherwise nearly insoluble materials in organic solvents. The scope of this method is further demonstrated by introducing a ruthenium(II) polypyridyl complex on the surface of the surface-bound AuNPs to generate fluorescent multicomponent materials.

INTRODUCTION The concept of building up hybrid systems is emerging as the dominant methodology to manufacture new materials maintaining the unique properties of the individual components, while expressing new functionalities.1-2 Hence surface functionalization of nanostructured materials is a challenging prerequisite for their real world applications. Controlled attachment of preformed nanostructures and small molecules to surface-inactive materials is hard to achieve.3-10 Two-dimensional (2D) nanomaterials based on transition metal chalcogenides (MS 2 ; M = W or Mo) have shown great potential for electrical and optical applications. No less important is the intensive research on these nanoparticles in the fields of sustainable and advanced

technologies, e.g. for catalytic processes; rechargeable batteries; polymer nanocomposites for supercapacitors, nanolubrication, etc. Various inorganic nanotubes (INTs) and fullerene-like (IF) nanoparticles, based on MS 2 , have been reported since their discovery in 1992.11,12 Detailed mechanistic studies of their formation and new design strategies resulted in large-scale production and commercialization. Some of these robust materials possess high pressure, impact, and heat resistances and are available as anti-wear and anti-friction additives for automotive, utilities, shipping, mining, aerospace and other industries.11 The reasonably low toxicity of IF/INT nanoparticles also opens up new opportunities for their medical applications.13 However, their relatively high chemical inertness makes it challenging to efficiently modify their electronic and optical properties or to generate hybrid materials with useful functionalities.

Figure 1. Schematic representation of the sequential functionalization of inorganic nanotubes and fullerenes with metallic nanoparticles and molecular components to generate hybrid materials with tunable bandgaps, enhanced solubility, and fluorescent properties.

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2H- MoS 2 closed shells (trigonal prismatic coordination) with an average diameter of 70 nm. Their synthesis was described

These nanostructures generally have few surface defects as well as low solubility, and hence it is particularly hard to functionalize them efficiently with organic or inorganic molecules.14 Thus, the formation and processing of hybrid materials from these inorganic nanostructures remains elusive. The chalcogenide layers in these inorganic nanostructures that sandwich the metal atoms (W or Mo) are generally inert. Moreover, they offer the metal atoms steric shielding from nucleophilic attack. Uniform and dense functionalization of the surfaces of metal dichalcogenides nanoparticles with other materials, such as 0Dmetallic nanoparticles, is a much sought after approach to induce functionality, but is rather difficult to achieve. One potential pathway for developing a standard surface chemistry for their functionalization is to introduce metals or molecules with high affinity for sulfur. Metallic nanoparticles (MNPs, M = Au, Ag, Pd) are known to bind with disulfides based on favorable soft-soft interactions according to Pearsons’s Hard Soft Acid Base (HSAB) Principle.5-10 Banerjee, Vajtai, and Ajayan used Lewis acid–base chemistry as an approach to control the electronic surface states of metal chalcogenides, without altering the overall structure of the host material.16 The use of MNPs serves a dual purpose: to impart metallic character to the semiconductor (INT-WS 2 ) surface and also to act as a mediator to attach ligands with various optical and electronic properties. The methods for the functionalization of such nanomaterials with MNPs available today are surface defect guided.5-10 Hence, several such attempts have resulted in sparse and/or non-uniform attachment of polydispersed MNPs on INT-WS 2 and IF-MoS 2 . Uniform and dense, yet controlled functionalization of these inorganic nanostructures is envisioned to regulate their physicochemical properties. In addition, the resulting hybrid materials could be subjected to post-synthetic modifications under carefully controlled conditions, to engineer luminescent multicomponent materials. We have recently introduced a method to efficiently coat INT-WS 2 with AuNPs and demonstrated the formation of freestanding, tubular AuNP-moleculare networks.15 In this paper, we report the sequential functionalization of the surfaces of transition metal chalcogenides (Figure 1): (I) Physical attachment of metallic nanoparticles (MNPs; M = Au,

before.12 The TOAB-capped AuNPs (∅ AuNP = 5 nm), AgNPs

(∅ AgNP = 17 nm), PdNPs (∅ PdNP = 2 nm), Citrate-capped AuNP (∅ AuNP = 12 nm), and [Ru(bpy) 2 (mbpy-pyNO)](PF 6 ) 2 were prepared according to published procedures.17-20 NaAuCl 4 ×2H2O, AgCl, and PdCl2 were purchased from Alfa Aesar. Sodium citrate monobasic and sodium borohydride were purchased from Sigma Aldrich. Tetraoctylammonium bromide (TOAB; 98%) was purchased from Sigma Aldrich and also from Chem-Impex International (TOAB; 99.4%), and was used as received. HRTEM images were obtained using a Tecnai F30 electron microscope (UT, FEI) operating at an accelerating voltage of 300 kV with a Gatan charge-coupled device (CCD) camera. TEM images were obtained with CM 120 (ST, Philips) operating at an accelerating voltage of 120 kV with a Gatan Ultrascan 1000 CCD camera. Energy dispersive X-ray spectroscopy (EDS) was performed on CM120 (ST, Philips) equipped with an EDAX Genesis EDS system. Scanning electron microscope (SEM) images were collected on an Ultra 55 FEG (Zeiss). UV-Vis spectra were obtained using Varian Cary 100 spectrophotometers (in double beam transmission mode). Absorption spectra of decoration of AuNP on INT-WS2 were obtained by measuring the amount of light absorbed using an integrating sphere Hamamatsu Quantaurus absolute QY system; calibration was done using single-pass absorption. Absorption data of decorated AgNP and PdNP on INT-WS2 and AuNP decoration on IF-MoS2 and temporal were obtained by Scatter-free absorbance spectra collected on a CLARiTY 1000 spectrophotometer (Bogart, GA, USA). The method for the determination of the value of Eg involves plotting (αhv)1/n against (hv), where hv is the energy of the incident photons, Eg is the value of the optical energy gap between the valence band and the conduction band, and n is the power, which characterizes the electronic transition, whether it is direct or indirect, during the absorption process in the K-space. Indirect transitions in many amorphous materials fit the case for n = 2; for a direct transition, a reasonable fit with n = l/2 is achieved.21 Raman spectra were obtained with a Renishaw Micro Raman Imaging Microscope with a 632.8 nm He-Ne laser. The laser power was 25 mW with a 10× objective used for focusing. The acquisition time was 10 s. The samples were drop-casted on a silicon wafer. Individual nanotubes were placed at 45° with respect to the XY plane. Scanning probe microscopy topography and scanning Kelvin probe microscopy were performed on a SmartSPM 1000TM (AIST-NT, Novato, CA USA). The conducting probes employed were ANSCM-PT (AppNano, Mt. View, CA USA) with a nominal resonance frequency of 60 kHz and a force constant of 3 N/m. Decoration of INT-WS2 and IF-MoS2 with Metallic Nanoparticles. (a) Reaction of INT-WS2 with TOAB-capped AuNPs

Ag, Pd) of various diameters (∅ AuNP = 5 nm; ∅ AgNP = 17 nm;

∅ PdNP = 2 nm) onto both INT-WS 2 and IF-MoS 2 . Tetraoctylammonium bromide (TOAB)-capped MNPs and the inorganic surfaces are both crystalline, resulting in high-yield and latticematched hybrid materials. The resulting hybrid materials have monodispersed coatings of a single layer of NPs, whose uniformity and density can be controlled. The optical band gap in the AuNP-containing materials is ~70 meV lower (compared to the non-functionalized INT and IF), and the contact potential distribution (CPD) measurements indicate incorporation of metallic characteristics to the hybrid material. (II) Decorating the surface-bound AuNPs with an alkylthiol enhances the solubility of the materials in organic solvents. Furthermore, new optical properties are achieved by introducing a ruthenium(II) polypyridyl complex.

(∅AuNP = 5 nm).

EXPERIMENTAL SECTION Materials and Methods INT-WS 2 was supplied by NanoMaterials Ltd, Israel. The length and diameter of INT-WS 2 are in the range 1-20 µm to 30-100 nm, respectively. The IF-MoS 2 are composed of ~30

Monophasic method with agitation:15 0.4 mg of INT-WS2 was suspended in 1.0 mL of toluene via sonication for 3 min. Then, this suspension was ice-cooled for 1 h and was reacted with an ice-cold toluene solution of TOAB-capped AuNPs (200


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µL; 2.67 mg/mL). The reaction mixture was sonicated for 1 min and kept for 30 min at 0°C. This step was repeated 3 times (600 µL of a AuNP was added in total). Monophasic method without agitation: INT-WS2 (0.4 mg) was dispersed in toluene (1.0 mL) via sonication for 3 min. Subsequently, the dispersion was transferred to a vial and a toluene solution (600 µl) of TOAB-capped AuNPs (2.67 mg/mL) was added. The reaction mixture was left unperturbed. The reaction progress was monitored by UV/Vis spectroscopy. After 140 min, the red reaction mixture became colourless and the functionalized INT-WS2 precipitated. The sample was prepared for SEM. The intermediate samples were also prepared at 20 min and 40 min to follow up the decoration process. Biphasic method: INT-WS2 (0.4 mg) was dispersed in toluene (1.0 mL) via sonication for 3 min. Subsequently, H2O (3.0 mL) was added and the mixture was sonicated for 2 min, and INT-WS2 was accumulated at the toluene-H2O interface. TOAB-capped AuNPs (2.67 mg/mL) in toluene (200 µL) was added dropwise to the toluene phase and the reaction mixture was sonicated for 2 min at 0°C. This step was repeated 3 times. Drop-casting method: A dispersion (20 µL) of INT-WS2 (0.4 mg in 3.0 mL of toluene) was drop-casted on a silicon substrate and dried under a gentle flow of N2. Subsequently an icecold toluene solution of TOAB-capped AuNPs (10 µL; 2.67 mg/mL) was added and allowed to react for 20 min. The formed AuNP/INT was washed with toluene (20 µL, 3×), and dried under a gentle flow of N2.

topography was recorded in the dynamic mode (amplitude modulation) and in the second pass the probe traced out the topography at an additional lift height of 10 nm above that at which the topography was made, while the modulation was induced electrostatically. In this second pass, the bias voltage between the tip and substrate was modified by the feedback system to keep the frequency constant (FM mode). This bias voltage was recorded and gives the changes in the contact potential difference between the tip and sample. Use of the FM mode in the SKPM measurement gives better spatial resolution than the amplitude modulation (AM) mode and is essential for getting an accurate estimation of the CPD over the nanotube with minimal averaging of the surrounding substrate. Since these values are relative, we reference all values to the silicon substrate. Conducting-Probe AFM (CP-AFM) Measurements CPAFM measurements were performed under similar conditions to those reported for SKPM measurements, except the substrate was an Au film (100 nm thickness) evaporated on the Si wafer. I-V measurements were performed on selected locations on the sample, guided by the topographical image made with the same probe in semicontact mode. Directly after acquiring the topographic image, I-V curves were made by a custom software routine which moved the probe to the selected location in semicontact mode, then made continuous contact under a pre-determined force, while recording the I-V curve (curve time several hundred ms). This protocol avoided lateral scanning of the tip while in contact with the surface which minimized damage both to surface and to tip. Despite this, during the course of the measurements, the tip could become contaminated so that no current was detected in a voltage sweep between ± 2 V. This could also be due to local contamination on the surface. Such curves were discarded. At the other extreme, when there was no appreciable resistance between tip and substrate, the I-V curves were Ohmic and reflected the 10 MΩ limiting resistor RL positioned between voltage source and tip. Reaction of AuNP/INT with Dodecanethiol. AuNP/INT (0.4 mg) in toluene (3.0 mL) was reacted for 1 h at 0ºC with a toluene solution of dodecanethiol (50 µL, 7.1 µg/mL). The nanotubes were then collected and analysed by SEM to confirm that the treatment with the thiol did not result in the release of the AuNPs. Spectroscopic analysis of the solution did not show free AuNPs. Formation of Ruthenium Trisbipyridine (N-oxide): [Ru(bpy)2(mbpy-py)NO](PF6)2. [Ru(bpy)2(mbpy-py)](PF6)222 (282 mg; 0.28 mmol) was dissolved in freshly distilled dichloromethane (100 mL) and the solution was degassed with argon for 15 min. Meta-chloroperoxybenzoic acid (mCPBA) (1.5 equiv., 82 mg, 0.42 mmol) was added at room temperature during the course of 15 min. The progress of the reaction was monitored by TLC. Upon completion of the reaction (~24 h), the complex was purified by preparative TLC (MeCN/H2O/sat. KNO3; 400/100/15 v/v/v). Acetonitrile was removed under reduced pressure, and the complex was precipitated by adding NH4PF6 (68 mg, 0.42 mmol). The complex was filtered off and dried overnight under high vacuum to yield the product (58%, 165 mg) as an orange solid. Single crystals suitable for X-ray crystallographic measurements were obtained by the diffusion of toluene into an acetone solution of the complex over a week. In a typical experiment, a vial with an acetone (3.0 mL) solution of the Ru-complex (30 mg) was placed in a larger sealable vial containing toluene (10 mL). The orange solution of complex turned light yellow after 7 days at room temperature and red

(b) Reaction of INT-WS2 with TOAB-capped AgNPs (∅AgNP = 17 nm). An ice-cold toluene dispersion of TOAB-capped AgNPs (1.0 mL; 0.46 mg/mL) was added to a dispersion of INT-WS2 (0.4 mg) in toluene (3.0 mL) and the mixture was sonicated (3×) for 1 min under ice-cold conditions. The resulting nanotubes (AgNP/INT) precipitated and were washed with toluene until no free AgNPs in the toluene were observed by extinction spectroscopy. (c) Reaction of INT-WS2 with TOAB-capped PdNPs (∅PdNP = 2 nm). An ice-cold toluene solution of PdNPs (1.0 mL; 1.76 mg/mL) was added to a dispersion of INT-WS2 (0.4 mg) in toluene (3.0 mL) and the mixture was sonicated (3×) for 1 min under ice-cold conditions. The decorated nanotubes (PdNP/INT) precipitated and were washed with toluene until no free PdNPs were observed in toluene by extinction spectroscopy. (d) Reaction of INT-WS2 with citrate-capped AuNPs (∅AuNP = 12 nm). INT-WS2 (0.4 mg) was dispersed in ethyl acetate (1.5 mL) by sonication for 3 min. Subsequently, citratecapped AuNPs in water (0.10 mg/mL, 2.0 mL) were added the mixture was subjected to a vortex for 10 s and shaken at 200 rpm for 6 h. The tubes accumulated at the ethyl acetate-H2O interface. SEM and HRTEM images are shown in Figure S1. Kelvin Probe Force Microscopy of Metallic Nanoparticles Decorated on INT-WS2. Dispersions of INT-WS2 (0.4 mg in toluene, 1.0 mL) as well as decorated nanotubes - AuNP/INT, AgNP/INT2, and PdNP/INT prepared by the monophase method were drop-casted on a silicon wafer and dried under a gentle flow of nitrogen after absorbing excess solvent with a Kimwipe. Subsequently, the sample was analyzed by AFM in a glovebox under dry nitrogen purge. The Kelvin probe measurements were made in a 2-pass mode: during the first pass the



crystals were formed at the bottom of the vial. Accurate mass ESI MS: m/z: 848.1277 (M+-PF6), 352.5768 (M2+-2PF6). UV/Vis (CH3CN), λmax = 485 nm. 1H NMR (500 MHz, C3D6O): δ 8.75 (2H, b), 8.78 (1H, s), 7.89 (4H, d; J = 5.9 Hz), 7.85 (1H, d; J = 16.4 Hz), 7.78 (1H, d, J = 16.4 Hz), 7.74 (2H, d; J = 5.8 Hz), 8.10 (3H, m), 8.17 (1H, d; J = 5.2 Hz), 7.6 (4H, m), 8.22 (4H, m), 8.83 (4H, d; J = 8 Hz), 9.07 (1H, s), 7.45 (2H, d; J = 5.9 Hz ), 2.61 (2H, s), 7.78 (2H, d; J = 5.4Hz), 8.1(1H, d). 13 C{1H} NMR (125 MHz, C3D6O): 20.31 (CH3), 138.04-121.26 (CH), 151.97 (Cq), 150.65 (Cq), 145.32(Cq) 151.85-150.90 (CH), 156.575- 157.75 (Cq), 148.72(CH). 19F{1H} NMR (376.7 MHz, C3D6O): δ -73 (d, J = 707 Hz). 31P{1H} NMR (162.07 MHz, C3D6O): δ -141 (septet, J = 707 Hz). Single Crystal X-ray Structure Determination of [Ru(bpy)2(mbpy-py)NO](PF6)2. Crystal data: C38H31N7ORu + 2(PF6) + 2(C2H4O2), red plate, 0.16 mm × 0.14 mm × 0.04 mm, triclinic P-1, a = 8.5990(5) Å, b = 13.7885(9) Å, c = 19.9990(11) Å, α = 102.697(5)°, β = 90.303(5)°, γ = 96.735(5)° from 3530 reflections, T = 100(2) K, V = 2296.1(2) Å3, Z = 2, Fw = 1112.81, Dc = 1.610 Mg•m-3, μ = 0.513 mm-1. Data collection and processing: Rigaku XtaLab diffractometer, Pilatus 200K detector, MicroMax003 MoKα (λ = 0.71073 Å), Confocal MaxFlux optics, -9 ≤ h ≤ 10, -16 ≤ k ≤16, -24 ≤ l ≤24, frame scan width = 0.50°, scan speed 1.0° per 60 s, typical peak mosaicity 1.5°, 23712 reflections collected, 8679 independent reflections (R-int = 0.1444). The data were processed with Rigaku CrysAlisPro. Solution and refinement: Structure solved with SHELXT-2013. Full matrix least-squares refinement based on F2 with SHELXL-201323 on 684 parameters with 251 restraints gave a final R1 = 0.0988 (based on F2) for data with I > 2σ(I) and, R1 = 0.1628 on 8679 reflections, goodness-of-fit on F2 = 1.026, largest electron density peak 1.553 e•Å-3, largest hole – 0.621 e•Å-3. C2H4O2 is a possible oxidation product of acetone.20 Reaction of AuNP/INT with [Ru(bpy)2(mbpypy)NO](PF6)2 for Correlative Fluorescence and Scanning Electron Microscopy. [Ru(bpy)2(mbpy-py)NO](PF6)2 (5.0 mg, 5.0 mmol) in DMF (100 µL) was added dropwise at 0°C under shaking to a dispersion of AuNP/INT (0.4 mg) in toluene (3.0 mL). After 20 min, the dispersion was washed (3 × 1.0 mL) with toluene/DMF (10:1 v/v) and the washings were analyzed by extinction spectroscopy. The samples were drop-casted on a marked glass substrate (ibidi IBD-CG000004), dried under a gentle flow of N2 and analyzed using a fluorescence microscope (Vutara SR200). The fluorescence emission was detected in the far red region (above 640 nm) when excited at λ = 488 nm. The same fluorescent regions were then observed correlatively by SEM.

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(Figure 2A-D) and IF-MoS2 (Figure 3A-D) are both uniformly covered with a monolayer of AuNPs with an average interparticle distance of 2 nm. The surface-bound AuNPs are monodispersed. Apparently, the TOAB-capping layer hampers the fusion of the surface-bound NPs with those in the dispersion. Further nucleation of additional layers of AuNP was not observed, which suggests that the sulphur-gold interaction predominates, i.e. a “van der Merwe” (self-assembly) growth model applies.24

Figure 2. Functionalization of INT-WS2 with AuNPs to form AuNP/INT. (A) HRSEM; (B) HRSEM (zoom in of A); (C) Backscattered SEM (D) TEM. For HRTEM images and Fast Fourier Transform (FFT) showing lattice matching see ref 15.

RESULTS AND DISCUSSION a) Uniform Decoration of INT-WS2 and IF-MoS2 with Metallic Nanoparticles. The reaction of INT-WS2 and IFMoS2 suspended in toluene with preformed TOAB-capped

Figure 3. Functionalization of IF-MoS2 with AuNPs to form AuNP/IF. (A) SEM, (B) high magnification SEM of a decorated AuNP/IF. (C) HRTEM, (C, Inset) Fast Fourier Transform (FFT) of the area marked in C at the interface of the AuNPs and IF showing lattice matching (circles), (D) TEM, and (E) high magnification image of the area marked in C at the interface of the AuNPs and

AuNPs (∅AuNP = 5 nm) at ice-cold conditions under sonication resulted in the formation of well-defined AuNP/INT15 and AuNP/IF, respectively. Alternatively, drop-casting an ice-cold solution of a toluene dispersion of AuNPs directly onto INTWS2 spread on a solid substrate yielded similar structures. The structural details of these new materials have been studied by electron microscopy (Figures 2-4). Scanning electron microscope (SEM) and transmission electron microscope (TEM) images showed that both INT-WS2

IF-MoS2. ∅AuNP = 5 nm. The white lines indicate the lattice continuation between the AuNPs (111) and IF-MoS2 (013).


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The monolayered and dense decoration is maintained, even in presence of a 5-fold excess of a toluene dispersion of AuNPs. The dispersions of AuNP/INT and AuNP/IF are stable for more than 6 months at 4°C. Some aggregation of the surface-bound NPs was found to occur at room temperature. High-resolution TEM imaging of the interfaces between the AuNPs and the inorganic structures revealed the structural details at the sub-nm level, which provided insights into the mechanism behind the observed functionalization. The interfacial lattice continuity between the (111) planes of AuNPs and the (013) planes of WS215 or (013) planes of MoS2 (Figure 3C,E) is clearly visible and ensured firm surface attachment. The lattice continuation in these hybrid composites is corroborated by the Fast Fourier Transform (FFT) of the HRTEM images, where two resolved spots originating from the two different components are clearly visible (Figure 3C, inset). The lattice mismatch between the AuNPs and INT-WS2 or IF-MoS2 is 640 nm). (B) SEM image of marked area 2 in (A). No fluorescence was observed for the nonfunctionalized AuNP/INT.

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